The title of Monday’s colloquium was “The Cryogenic World of Triton”, and the speaker was Gary Peterson of the Department of Geological Sciences, San Diego State University. Here he is in action:
The talk was very qualitative. Perhaps at times too qualitative for a lot of the Physicist audience, I’ve gathered from comments, but everyone agreed that it was still very interesting. Some of the qualitative aspects were necessary, since there’s not much data available for the sorts of things he wanted to talk about. So he was extrapolating and conjecturing a fair amount, not without interesting conclusions here and there. His abstract:
Triton is a large satellite in retrograde orbit around Neptune, the most distant (40AU) of the giant gaseous planets. Surface temperatures average about 40K and are cold enough to condense all of the heavier gases, including nitrogen. A pronounced tilting of Triton’s spin axis gives rise to a strongly seasonal climate and the available imagery indicates a deteriorating south polar cap made of nitrogen. Evidently the released nitrogen is being transferred to the north pole and condensing there during the long winter season. Distant though the sun is, it evidently supplies enough energy to provide seasonal transfer of nitrogen much in the same manner that water alternately collects and melts at the earth’s poles with the seasons. Triton has been resurfaced and shows little evidence of cratering. Probably the satellite has been heated, evaporating the surface ices and then the released gases have recondensed to provide a new surface. The energy source for the heating would appear to be tidal friction following capture by Neptune. The retrograde orbit of Triton would indicate capture and the lack of surface craters would suggest that the capture was a fairly recent event. Numerous irregular surface textures indicate repeated expansion and contraction and are compared with somewhat similar features on earth. Although the Earth/Triton surface features resemble one another, they would be composed of totally different material.
I found that there were several interesting aspects of that talk. Among those were: The fact that Triton is likely a captured satellite (it orbits Neptune in the wrong direction), and may be a Kuiper belt object originally, like Pluto, Charon, etc. The surface is very active (in geological terms – you can see this from the surprisingly low number of visible impact craters in the surface), and so this needs an energy source. The most likely source is internal heat generated by tidal activity due to its orbit of Neptune. This is likely enough (he thinks) to help with the process of heating, flowing, etc., taking place with the nitrogen on the surface.
Peterson’s central thesis is that the nitrogen is playing a similar role to that of water here on the earth’s surface, with regards some of the features you can see on the surface of Triton. He uses Yellowstone National Park as his model for several features, showing us pools of water surrounded by ice/snow, with steam rising out of them due to heating from the earth below. The surface temperatures of Triton are such that you have a lot of the same things going on, but with water replaced by nitrogen. He also spent some time looking at comparisons between various surface morphological features on Triton with those you can find in lots of local geological features (see photo at top). He used the “Devil’s golf course” in Death Valley as one of his examples. This was amusing to me since last Spring I’d been planning to do a post or two on CV on the patterns you can get in drying mud, based on my last trip to Death Valley. You may recall that I was near the sand dunes. I intended to do a post on some shapes I saw in the sand dunes, and also some of the lovely mud shapes that were right nearby. Circumstances changed, and so I did not do the post in the end, so I’m glad to be reminded of those photos. I’ll dig them out sometime, since I’ve some thoughts about shapes and length scales that you might like to read about.
Peterson seems to devote a lot of time to giving talks about aspects of geology as applied to planetary systems. I am not in a position to judge the overall effectiveness and impact of his work (comments welcome from any experts reading), but he does have a lot of interesting ideas. (See his “Independent Planetologist” site for what looks like hundreds of talks!) One of the most interesting was in response to a question from Geraldine Peters (from our space sciences center) about what he thinks the New Horizons mission will see when it gets a better look at Pluto. He expects that both Pluto and Charon will have surprisingly active surfaces, just like Triton, and for the same reasons. Internal tidal energy will have kept a lot of geologically interesting processes alive: They’re not just dead balls of ice.
Some Related Asymptotia Posts (not exhaustive):